Colours of Light: Wavelengths & Frequencies

[Big-Daddy]

Can all the colours of light that we can observe and distinguish as different colours of light - except white (a mix of all visible wavelengths), black (the absence of any visible wavelength) and greys (? - I'm guessing they're a mixture of black and white, though how it works I'd love to hear, seeing as white is presence of all wavelengths and black is absence of all wavelengths) - be described by specific wavelength (or frequencies) of the photons involved in light of that colour? If I pick a random object (which is not white, black or grey) and tried to describe its colour, would there be a wavelength of light which exactly represents that colour, or is the colour a mixture of wavelengths?

 

[D H]

The answer to your question is "no". The quintessential example is purple. Purple, unlike violet, is not a spectral color.

There have been many threads on this very topic. Look at the related discussions, listed at the bottom of the page.

 

[dauto]

Gray is the same thing as white. The difference between them is a matter of brightness, not hue of saturation.

 

[phinds]

Can all the colours of light that we can observe and distinguish as different colours of light ...

Uh ... stop right there. Even aside from things like that pointed out by HH, who is "we". My understanding is that different ethnic groups have different degrees of ability to "perceive" color variations based on the effect of things in their environment. For example, Inuits supposedly have a greater degree of differentiation (and a much larger number of names) for what most of us in the continental US would call "white". This is because snow and ice have a lot of different form and it matters to them in a way that it does not to us.

Likewise, I have read that some American Indian tribes developed distinctions among reds that whites don't have.

I don't mean in either of those examples, that their eyes are different than other humans but rather that your use of the word "distinguish" is ambiguous at best.

I was told by an avid collector of stamps that he had his color vision checked because the thought it was faulty because he was completely unable to distinguish subtle differences in pinkish red colors in early American stamps whereas his brother in law, who was American Indian, could tell them apart at a glance.

Color IS about wavelength but it is also about human perception which is not so easily quantifiable.

 

[sophiecentaur]

Color IS about wavelength but it is also about human perception which is not so easily quantifiable.

We must avoid the trap of getting into colour = wavelength (we were here only a week or so ago and we did it all to death).
It would not be overstating things to say that we never see any sources of light, in every day life (objects or illuminants) that consist of just one wavelength. Except in the Lab, all the things we see have complicated spectra (a mixture of wavelengths at different levels) and our very limited visual system groups what we see into a rather muddled set of mutually agreed (but not universally so) and commonly named 'colours'.

Grey and White are both the same thing (spectrum) - just with different intensities. Black is just the absence of any visible light. There are as many 'whites' as you care to define, because they all depend upon the form of illuminant - Sun / cloudy sun / tungsten (hot) / tungsten (warm / fluorescent - your automatic camera has a few options in its colour balance adjustment.

The whole business of colourimetry is very complicated and nothing they tell you at school or, probably, even in art college is about the phsycophysics of colour vision.

 

[phinds]

We must avoid the trap of getting into colour = wavelength

Good point. Thanks for that correction.

 

[Khashishi]

My understanding is that different ethnic groups have different degrees of ability to "perceive" color variations based on the effect of things in their environment.

Do you have any sources for that understanding? It's true that different cultures have different words for colors, but I was not aware of any difference in sensation.

 

[phinds]

Do you have any sources for that understanding? It's true that different cultures have different words for colors, but I was not aware of any difference in sensation.

Sadly, I do not. I read some stuff about it a couple of decades ago. In (somewhat) direct experience, I have only the anecdote above about the American Indian.

Again, I am not saying that there is any difference in physical ABILITY to make such distinctions, I am emphasizing that training matters in the development of such abilities, so I do believe the difference is in more than just words (but, again, I cannot back that up with any reference).

 

[sophiecentaur]

This is not surprising to me. I think it is probably something along the lines of musicality. It's something that you have a basic innate amount of but your training can enhance what you have in certain directions. People can develop perfect pitch and I think they can develop good colour memory and discrimination if their job or culture encourages it.

 

[phinds]

This is not surprising to me. I think it is probably something along the lines of musicality. It's something that you have a basic innate amount of but your training can enhance what you have in certain directions. People can develop perfect pitch and I think they can develop good colour memory and discrimination if their job or culture encourages it.

Yes, my point exactly. I don't find it surprising at all.

 

[sugeet]

Anyways since plane waves are hypothetical, everything we see is finally a small band of wavelengths!

 

[sophiecentaur]

I don't think you are referring to plane waves. Do you mean continuous waves?

 

[sugeet]

I don't think you are referring to plane waves. Do you mean continuous waves?

If you are referring to me, then yes I mean plane waves only, because plane waves are supposedly single frequency and they are hypothetical, we always have a finite width and not a Dirac Delta function in the frequency domain.

 

[sophiecentaur]

If you are referring to me, then yes I mean plane waves only, because plane waves are supposedly single frequency and they are hypothetical, we always have a finite width and not a Dirac Delta function in the frequency domain.

This isn't correct. The word 'plane' refers to the geometry of the situation and not to the spectrum. It refers to a situation where the wave front is a plane. The wave can consist of a single pulse (infinite bandwidth) or cw (single frequency) yet still be plane. If the distance between source and detector is great enough then the wave, in a particular direction, will have a spherical wave front which, as r tends to infinity, approaches a plane.

 

[sugeet]

This isn't correct. The word 'plane' refers to the geometry of the situation and not to the spectrum. It refers to a situation where the wave front is a plane. The wave can consist of a single pulse (infinite bandwidth) or cw (single frequency) yet still be plane. If the distance between source and detector is great enough then the wave, in a particular direction, will have a spherical wave front which, as r tends to infinity, approaches a plane.

Plane wave is defined as exp(-iwt), where 'w' is the frequency. In Fourier decomposition when we say that any complicated wave-function can be expressed as a superpostion of plane waves, we then do not refer to the geometry of wavefronts but to the meaning of a specific frequency. Plane waves by definition are of specific constant frequency and infinite bandwidth, they are hypothetical, mathematical abstraction.

 

[sophiecentaur]

Plane wave is defined as exp(-iwt), where 'w' is the frequency. In Fourier decomposition when we say that any complicated wave-function can be expressed as a superpostion of plane waves, we then do not refer to the geometry of wavefronts but to the meaning of a specific frequency. Plane waves by definition are of specific constant frequency and infinite bandwidth, they are hypothetical, mathematical abstraction.

We must be discussing in different contexts. I am using the definition as in this link where the 'plane' terms refers to the geometry. (The surface of constant phase of a plane wave is a plane.) That's how I have always seen it used in RF and Optics. Have you a reference where I can see the expression used 'your way'? These things often morph in different directions in 'meaning space'.
I have a problem with this because, conventionally, a constant frequency signal has Zero Bandwidth and infinite temporal extent. Are we in different Universes? lol.

 

[sugeet]

Oh sorry, I mean constant frequency and zero bandwidth, I am sorry, that's a mistake, I mean it extends infinitely in temporal space.

 

[sophiecentaur]

OH- right (panic over!)
But the term 'plane wave ' refers to geometry as well as time.
Your "exp(-iwt)" only refers to a time varying signal and not a wave. You need an spatial vector involved, as in exp(-i(wt - kx)), to describe a wave traveling along the x axis.
I know that, in RF circles, people talk about 'CW' (meaning Carrier Wave or Continuous Wave) but that sort of assumes it's been transmitted through space and received and it's a teeny bit sloppy Engineers' talk.